In recent years, as vehicles have become quiet, vehicle noises caused by the rolling load of a pneumatic tire have become a large factor of vehicle noises, and there is a demand for a reduction of such noises. Tire noises of high frequency, particularly of around 1000 Hz have become a main cause of vehicle exterior noises, and there has been a demand to reduce such tire noises. This was also desired from the viewpoint of addressing environmental issues.
Tire noises of around 1000 Hz are generated mainly by air column resonance. Air column resonance sound is a noise generated by resonance of air in a tube defined by a circumferential groove continuously extending in the tread circumferential direction and the road surface. In general vehicles, the sound is generated in the range of around 800 Hz to 1200 Hz, and due to the high sound pressure level at the peak and the wide frequency range, it is a large part of the noises generated by a pneumatic tire. Further, since the human hearing sense is especially sensitive to noises in a frequency band of around 1000 Hz, a reduction in such air column resonance sound is also effective in terms of improving quietness felt by drivers at the time of travelling.
As pneumatic tires with reduced air column resonance sound, for example, a pneumatic tire provided with, in the land part defined by circumferential grooves, a resonator having an air chamber opened to the land part surface and one narrowed neck communicating the air chamber to a circumferential groove (e.g. PTL 1 (JPH05338411A)), and a pneumatic tire provided with, in the land part defined between two circumferential grooves, a resonator having an air chamber opened to the land part surface, one or more narrowed necks communicating the air chamber to one of the two circumferential grooves, and one or more narrowed necks communicating the air chamber to the other circumferential groove (e.g. PTL 2 (JP2007269144A)) have been proposed. It is disclosed that, according to the former pneumatic tire, disposing a resonator would enable reducing air column resonance sound generated in a circumferential groove. Further, it is disclosed that, according to the latter pneumatic tire, since air column resonance sound generated in two circumferential grooves can be reduced together, the number of resonators to dispose can be reduced while effectively reducing air column resonance sound compared to the resonator having a narrowed neck opened to one circumferential groove of the former pneumatic tire, and therefore a decrease in land part rigidity can be prevented.